1. FIELD OF THE INVENTION
The invention relates to heat-treatment of semiconductor wafers such as in the diffusion apparatus and the chemical vapor deposition apparatus, and more particularly to an apparatus and a method of heat-treatment adapted for heat-treating one or two wafers at the same time, with uniformity, in a short time period.
2. DESCRIPTION OF THE RELATED ART
In a semiconductor device, the so-called "semiconductor function" is exhibited by introducing a minute amount of atoms of boron, phosphorus arsenic, etc. as impurities in a highly pure solid state substrate, such as a silicon wafer. Impurity atoms are introduced into a substrate in an ion implanting apparatus and then the substrate is heated at 800.degree.-1200.degree. C. for a predetermined time period in an atmosphere of nitrogen gas, argon gas, hydrogen gas etc. in a diffusion apparatus to diffuse the impurity atoms by a predetermined depth with tolerable dispersion and to cure the crystal defects caused by the ion implantation.
Also, in the case of forming an oxide film on a substrate, e.g. a gate oxide film of a MOS transistor or an inter-element isolation oxide film, a substrate is heated in an atmosphere of oxygen gas or steam in a diffusion apparatus to form a thermal oxide film with a tolerable thickness dispersion.
In the heat treatment apparatus used in the conventional diffusion process, a multiplicity of semiconductor wafers are arranged on a quartz boat and are inserted into a horizontally placed cylindrical furnace.
In recent years, it has been desired to form shallow diffusions and thin oxides uniformly over the whole surface of a semiconductor wafer by a short time heating corresponding to the miniaturization of patterns. As a heat treatment apparatus for satisfying such requests, there is proposed a structure, as disclosed in the specification of JP-A-60-171723, in which a vertical cylindrical high temperature furnace has an opening at a lower end and a horizontally supported wafer, one for each time, is inserted into the high temperature furnace from the lower end opening to heat the wafer.
This conventional technique, however, does not permit simultaneously and uniformly heating two wafers in a short time period. Heating the wafers one by one results in poor productivity. If two wafers are to be simultaneously heated, they are placed side by side horizontally. Then, there will be generated very large temperature differences from wafer to wafer and in one wafer. Further, if a wafer is heated for a short time period and rapidly taken out of a high temperature furnace, the very hot wafer will be directly exposed to the external atmosphere and may be contaminated thereby. Yet further, the afore-mentioned technique does not allow for heating a large wafer uniformly, which will be required by recent trend of further scaling down the patterns and employing wafers of a larger diameter.